Redundant speed control for brushless hall effect motor

ABSTRACT

A speed control system for a brushless Hall effect device equipped direct current (D.C.) motor in which separate windings of the motor are powered by separate speed responsive power sources and in which a change in speed, upward or downward, because of the failure of a component of one of the power sources results in a corrective signal being generated in the other power source to supply an appropriate power level and polarity to one winding to cause the motor to be corrected in speed.

O United States Patent 1 1 1111 3,757,183

Nola Sept. 4, 1973 1 REDUNDANT SPEED CONTROL FOR 3,461,367 8/1969Takeyasy 318/138 BRUSHLESS HALL EFFECT MOTOR [75] Inventor: Frank J.Nola, Huntsville, Ala. P imary Examiner-Harold Broome [73] Assignee: TheUnited States of America as mstan mmmer Omas Langer Filed: Mar. 30, 1972Appl. No.: 239,574

Att0rney-L. D. Wofford, Jr. et al.

[57] ABSTRACT A speed control system for a brushless Hall effect deviceequipped direct current (D.C.) motor in which separate windings of themotor are powered by separate speed responsive power sources and inwhich a C(il. 3l8l2fi4ziil change in Speed, upward or downward, becauseof the [58] Fie'ld 327 328 failure of a component of one of the powersources rei sults in a corrective signal being generated in the otherpower source to supply an appropriate power level and I 56] ReferencesCited polariitv to onej winding to cause the motor to be correc e 1n see UNITED STATES PATENTS p 3,383,574 5/1968 Manteuffel 318/138 6 Claims,2 Drawing Figures SPEED "MM SENSOR 2Ou SPEED SENSOR l POWER CONTROL 2446 E a as I l 3- 1 1. r 1 wwcommunion I I H MN w l l l li I v 40% I "M"H 111:1| so I l V, 1 I 240 46a 2 48a ,5811 I i \40 1 50a COMPARATORBUFFER emu 2 523: I

AM? SET swlTcH 1 L 7 POWER CONTROL REDUNDANT SPEED CONTROL FOR BRUSHLESSHALL EFFECT MOTOR ORIGIN OF THE INVENTION The invention described hereinwas made by an employee of the United States and may be manufactured andused by or for the Government of the United States of America forgovernmental purposes without the payment of any royalties thereon ortherefor.

BACKGROUND OF THE INVENTION This invention relates to systems for thecontrol of the speed of electrical motors and particularly to animproved system for the control of the speed of Hall effect deviceequipped brushless D.C. motors.

GENERAL DESCRIPTION OF THE PRIOR ART Conventional direct current motorsrequire commutators to supply currents to motor armature windings in thecorrect, and periodically reversible, polarities to maintain torque onthe armature of the motor. Such commutation is typically provided bymeans of a wearing contact between a copper commutator and carbonbrushes. This arrangement suffers the disadvantages that brushes must beperiodically replaced and undesired sparking between brushes and thecommutator frequently occurs. In an effort to overcome thesedisadvantages it has been previously proposed that commutation beachieved by the use of Hall effect devices which under the influence ofa permanent magnet rotor cause the direction of motor winding currentsupplied through them to periodically change in accordance with thetorque required of the windings for a particular orientation of therotor. Such an arrangement is described in Manteuffel et al. US. Pat.No. 3,165,685.

The control of the speed of D.C. motors commutated by Hall effectdevices has heretofore involved the rather straightforward technique ofobtaining a speed responsive signal from a tachometer driven by themotor, comparing the output of the tachometer with a selected referencevoltage representative of a desired speed, and then using the differenceor error signal to control the power fed to all windings of the motor.While for many applications this system of control is adequate it hasbeen found that where extreme reliability is required that systemcomponent failures remain a problem. Such failures can produce eithertoo large or too small control signals, or produce error signals of thewrong polarity, causing the speed of the controlled motor tosubstantially deviate from a desired speed.

SUMMARY OF THE INVENTION Accordingly, it is an object of the presentinvention to provide a motor control system for Hall effect devicemotors with improved reliability.

The motor speed control system of this invention encompasses separatecontrol and power drive systems, or control channels, for each of twowindings of a Hall effect motor. Each control system includes acomparator in which the actual speed signal from a tachometer iscompared with a desired speed signal to produce a speed error signal, abi-directional complementary circuit buffer amplifier for amplifying theresulting speed signal through which the error signal is fed to the Halleffect device providing reverse-current of power for one of thewindings, and a reverse-crine shutoff circuit to minimize the effect ofa failure of the speed error signal in the reverse current direction.

In the event that there occurs a component failure in one of the controlchannels such that a zero input or maximum forward speed input isundesirably applied to one of the windings of the motor, the othercontrol channel will provide an appropriate decreased or reversed inputto the other winding and cause the speed to be maintained. In the eventthat there is a circuit fail ure in one of the control channels whichproduces a faulty signal which tends to apply an undesired large ormaximum reverse current to one of the Hall effect devices and thus anundesired large or maximum reverse torque to the motor, this is sensedby the reverse current shutoff circuit and the faulty signal is reducedto a small insignificant value permitting the other channel tosufficiently compensate to correct or maintain a desired motor speed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an electrical block diagramillustrating the system of the invention.

FIG. 2 is an electrical schematic diagram illustrating certain detailsof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates the inventionin block form and FIG. 2 provides details of the circuitry employed.Referring to the drawings, this invention encompasses two identical, andredundant, control systems 10 and 12, one supplying power to winding 14and the other to winding 16 of Hall effect brushless D.C. motor 18. Inview of the identity between control systems 10 and 12, only controlsystem 10 will be described in detail. Like circuits and components ofthe control systems bear like numerals with major functional circuitsand components of control system 12 being further identified with thesubscript a.

Speed sensor 20, an AC. generator, is mechanically coupled to permanentmagnet annature or rotor 22 of motor 18 and it provides an AC. signaloutput, or speed signal, which varies in amplitude and frequencydirectly proportional to the speed of rotor 22. This speed signal is fedto comparator 24 where it is adjustable in amplitude by means ofpotentiometer 26 (FIG. 2) and is fed through rectifier 28 to impresspositive half wave pulses on capacitor 30 with respect to common ground32. Capacitor 30 integrates or smooths the positive pulses and providesa positive D.C. input through resistor 34 to the inverting input ofdifferential amplifier 36. The non-inverting input of differentialamplifier 36 is connected through rectifier 38 to a positive terminal ofadjustable D.C. reference voltage source 40 which produces a selectedvoltage, or demand signal, representative of desired motor speed.Rectifier 38, which is identical to rectifier 28, provides an equalizingtemperature-sensitive impedance with respect to rectifier 28 to maintaincircuit balance despite changes in impedance caused by variations inenvironmental temperature.

Differential amplifier 36 and the circuitry directly connected to it,making up comparator 24, function to compare the the amplitude of thespeed signal and the reference, or demand signal. When the speed signalis less than the demand signal, which will be the case when motor 18 isinitially started and until it reaches a desired speed, as representedby the demand signal, the output of differential amplifier 36 andcomparator 24 will be positive. In the event that the motor speedexceeds a desired speed, indicated by the speed signal being greaterthan the demand signal, the output of differential amplifier 36 andcomparator 24 will be negative.

The output of comparator 24 (FIG. 2) is fed to the base inputs of PNPand NPN complementary transistors 42 and 44, respectively, making upbuffer amplifier 46. These transistors are conventionally connected to apower supply, not shown, which provides plus and minus polarities withrespect to common ground 32. The emitter connected output of bufferamplifier 46 is connected through gain setting circuit 48 across acontrol circuit input of Hall effective device 50 of motor 18. Gainsetting circuit 48 consists of resistor 51 in parallel with resistor 52and diode 54 which are in series. With the diode polarity as shown, andin the event of a negative output on the emitters of transistors 42 and44 of buffer amplifier 46, the current flow through Hall effect device50 would be greater than where there is a like value of positive voltageon the emitters of transistors 42 and 44. The purpose of this differencewill be explained below.

The output of buffer amplifier 46 is coupled through resistor 56 to theinverting input of differential amplifier 36 to provide a negativefeedback path around buffer amplifier 46 and differential amplifier 36to regulate gain and improve the response of both of these amplifiers.The output of buffer amplifier 46 may be considered as being coupled toHall effect device 50 through failure sensitive switch 58 whichfunctions to substantially remove any input to Hall effect device 50 inthe event that it senses an abnormally high negative, reverse, input toHall effect device 50. Failure sensitive switch 58 employs transistor60, the emitter-collector terminals of which are connected across Halleffect device 50 and the base input of which is connected through 7 voltZener diode 62 to the output of buffer amplifier 46. Zener diode 62 andcurrent limiting resistor 61 causes transistor 60, a PNP type, to beenergized only in the event that the output of buffer amplifier 46should become negative and of a magnitude of at least seven volts, thebreakdown voltage of Zener diode 62.

Assume, for example, that the supply voltages to transistors 42 and 44are both 12 volts, positive and negative, respectively, and that thevalue of resistors 51 and 52 are of equal value. Thus with transistor 42on and transistor 44 off and a positive output voltage from bufferamplifier 46, the resulting current flow to Hall effect device 50 wouldbe half that if the opposite were the case. In other words the samemagnitude of current flow to Hall effect device 50 would be obtainedwith transistor 42 full on providing a full 112 volt output as withtransistor 44 only half on providing a 6 volt output. Hence, in thereverse torque mode, with a negative output, the control effect ortorque gain of the system on motor 18 is substantially greater. Zenerdiode 62 is then chosen to have a value just in excess of 6 volts or 7volts. Now if due to a circuit failure a full positive value of 12 voltsappears at the output of one of buffer amplifiers 46 or 46a, the othercan fully compensate by providing a negative 6 volt output. On the otherhand if due to a circuit failure in one of the buffer amplifiers or thepreceding circuitry, the output voltage should go to a negative voltageof 12, or between 7 volts and 12 volts and thus there is a voltage onZener diode 62 in excess of the breakdown voltage of 7 volts, transistor60 will be turned full on and the voltage across the Hall effect devicein circuit with it will be reduced to substantially zero. This will thenenable the other channel to provide a small positive compensating outputto maintain-the speed of motor 18 to the required speed.

The output voltage of Hall effect device 50 is supplied throughresistors 64 and 66 of summing circuit 68 to the inputs of differentialamplifier 70 of power control 72.

Power control 72 provides a control power input to winding 14 of motor18 of a magnitude and polarity determined by comparator 24 and thesinusoidal magnetic field applied to Hall effect device 50 by rotor 22(FIG. 1). The current demand input of differential amplifier 70, fedfrom Hall effect device 50, is algebraically summed by summing circuit68 with voltages across resistors 72 and 74 indicative of actual currentflow through winding 14 as obtained across resistors 76 and 78 in serieswith winding 14. The resulting net input to differential amplifier 70 isrepresentative of the difference between the summed inputs. The outputof differential amplifier 70 is applied to pulse-width modulator 80which provides a train of constant amplitude pulses out of output 81 orout of output 83 depending on the polarity of the output of amplifier70. The pulses vary in width directly with the magnitude of the outputof differential amplifier 70. Pulse-width modulator 80 is generally ofthe type described in U.S. Pat. No.

The output of pulse-width modulator 80 is applied to switch control 82or switch control 84 which control switching of transistors 86 and or 88and 92, respectively, of bridge switching circuit 4 which controls theflow of current from a positive supply terminal 96 through winding 14 toground in a direction determined by the polarity of output ofdifferential amplifier 70. With a zero input, switch control 82 appliesan on" signal to transistor 90 and an off signal to transistor 86 andswitch control 84 applies an on" signal to transistor 92 and an offsignal to transistor 88. With a positive pulse input, switch control 82applies an off signal to transistor 90 and an on signal to transistor86. Switch control 84 is not operated when switch control 82 is beingoperated. Thus a positive output from summing amplifier 70 produces apulse train out of output 83, of pulse-width modulator 80 which operatesswitch control 82 causing current to flow from left to right throughwinding 14. Similarly a negative output from summing amplifier 70produces a pulse train out of output 81 of pulse-width modulator 80which operates switch control 84 and causes current to flow from rightto left in winding 14. By virtue of the periodic reversal of output orcommutation by Hall effect devices 50, a continuous torque independentof rotor position is effectively applied in a desired direction toeffect rotation of rotor 22. Diodes 98 and 100, connected across thecollector-emitter circuits of transistors 90 and 92 permit current flowthrough winding 14 between pulses when transistors 86 and 88 are bothturned off to thus produce an essentially continuous winding currentflow from the applied variable width current pulses from the powersource. This characteristic is produced by the naturally inductiveeffect of winding 14. The winding current then is sinusoidal withamplitude and phase controlled by the sinusoidal output of Hall effectdevice 50. Identical power control 72a functions in the same manner toapply power to winding 16 of motor 18.

OPERATION In describing the operation of the illustrated system, exceptwhere necessary and convenient to refer to components of both controlsystems and 12, only control system 10 will be referred to. It, ofcourse, is to be understood that the operation of control system 12 isidentical. Assume initially that rotor 12 of motor 18 is at rest.Reference voltage source 40 would be adjusted to provide an outputcorresponding to a desired speed from motor 18. Variable speed resistor26 would be set to provide the same voltage at the desired speed assource 40.

With a zero output from speed sensor there would be applied todifferential amplifier 36 only a noninverting terminal input and thusthere would be provided a positive output from differential amplifier24. This positive output would be applied to the base input of bufferamplifier 46 turning transistor 42 on and transistor 44 off and thusproviding a positive output which would cause a current to flow in aforward direction, left to right, through resistor 51 and Hall effectdevice 50 to ground. Zener diode 62 although being forwardly biasedwould have no effect because the base of PNP transistor 60 would have areverse input bias and thus failure sensor switch 58 remains open orinoperative.

The output voltage of Hall effect device 50 is proportional to thecurrent flowing through the device and the magnetic flux from rotor 22passing through it.

Depending upon the position of rotor 22 and thus the polarity of thesinusoidal magnetic flux acting on Hall effect device 50 there will beprovided an appropriate sinusoidal input to differential amplifier 70 tocause it to provide the correct polarity for a desired direction ofrotation of motor 18.

When the output voltage of speed sensor 20 is lower than the referencevoltage of source 40 comparator 24 will be in a full positive state,causing Hall effect device 50 to be fed constant excitation or inputcurrent. The resultant output of Hall effect device 50 is a constantamplitude sinusoid. It will be assumed that the peak value of thissinusoid voltage represents a peak current demand for each of thewindings of approximately 10 amps.

The sinusoidal output voltages of Hall effect devices 50 and 50a areapplied to resistors 64 and 66 and 64a and 66a of amplifiers 70 and 70a.These voltages are amplified by amplifiers 72 and 72a causing sinusoidalcurrents to flow in windings 14 and 16. The winding current is sensed byresistors 76, 78 and 76a, 78a and is fed back through resistors 72 and74 and 72a and 74a where it is subtracted from the voltage applied bythe Hall effect devices. Hence, the current feedback forces the windingcurrent to be exactly at the value commanded by the Hall effect devicevoltage regardless of changes in winding resistance, counterelectromotive force, and battery voltage. Thus the current is controlled so that no surge current occurs at initial turn-on or when rotorspeed and counter E.M.F. are low. The peak value of the Hall effectdevice voltages when negatively summed with the feedback voltages insumming circuit 68 and 68a, whose input and feedback resistors are ofthe proper ratio, will result in peak currents of 10 amps in windings l4and 16.

As rotor 22 commences to rotate a counter E.M.F. is developed in thewindings which tends to reduce winding current to less than 10 amps, andthus create less torque. This reduction in current reduces the voltageacross resistors 76 and 76a and 78 and 78a and thus reduces the feedbackvoltage, causing the net input signal on differential amplifiers and 70ato increase until the desired peak current of 10 amps, and thus thedesired torque, are restored. This, of course, occurs almostinstantaneously.

In this manner the speed of rotor 22 and its inertial load are increasedby the application of a relatively constant torque until the desiredspeed, as determined by the settings of reference voltage sources 40 and40a, is reached. As the speed nears the desired speed and thus theoutput of speed sensors 20 and 20a approach the voltages of referencesources or inputs 40 and 40a comparators 24 and 2 40 provide smalleroutput voltages. These in turn cause smaller voltages to be applied byHall effect devices 50 and 50a to differential amplifiers 70 and 70a andthus cause the pulse width of pulses of pulse-width modulators 80 and80a to decrease in width to decrease the commanded currents applied towindings l4 and 16. At the desired speed the reference inputs 40 and 40aand the speed feedback inputs to differential amplifiers 36 and 360 willbe nearly balanced. The small error signal developed by the differencein these two signals drives the outputs of amplifiers 36 and 36a tomagnitudes which will maintain the current in windings l4 and 16required to overcome the frictional torques being applied by the load. Adecrease in speed caused by an increase in load torque is sensed byspeed sensors 20 and 20a and causes comparators 24 and 24a to driveharder. Similarly an increase in speed caused by a decrease in loadtorque will cause comparators 24 and 24a to reduce their outputs. Inthis fashion rotor 22 is maintained at a constant speed.

As previously observed a purpose of this invention is to enable speedcontrol to be maintained despite the failure of substantial portions ofthe system. To accomplish this, it was determined that some means mustbe provided to reduce speed, other than simply by cutting off power, inthe event that an excessive demand signal arises in some fashion in thesystem because of component failure. This was accomplished in thepresent system by the inclusion of bi-polarity control circuits such asbuffer amplifier 46 and 46a and pulse-width modulator 80 and 80a whichenable negative and positive speed errors to be corrected by theapplication of bidirectional torques. Thus for a positive direction ofcurrent flow in Hall effect devices 50 and 50a, output signals areprovided to apply torque in one direction. With a negative direction ofcurrent flow in Hall effect devices 50 and 50a, output signals areprovided to provide a storage in the opposite direction.

By then utilizing two such speed controls, one controlling power to onewinding and the other controlling power input to the other winding of aDC. motor, provision is made to compensate for a component failure ineither control which tended to produce either an over-speed condition oran under-speed condition.

In creating the capability for correcting overspeed as well asunderspeed, it was determined that the sensitivity of the overspeedcontrol signal, negative signal, ap-

plied to Hall effect device 50 should be nearly double the sensitivityof the underspeed, positive signal, applied to Hall effect device 50. Inother words, if a maximum slow down or negative signal is applied to theemitters of transistors 42 and 44, a maximum output voltage,substantially the source voltage, of minus 12 volts appears on theemitters of transistors 42 and 44. If this output were simply permittedto be impressed through only resistor 51 on Hall effect device 50 thedecelerating torque provided by winding 14 plus the frictional torqueapplied by the load would be greater than the accelerating torque thatcould be provided by winding 16 if there is a maximum positive inputsignal of 12 volts on the emitters of transistors 42a and 44a. Thus inthe event that there occurred a component failure which would create amaximum negative torque in control systems or 12, the friction torqueapplied by the load would aid the decelerating torque and it would beimpossible for the other one, by the application of its maximum positivetorque to compensate for this condition and motor 18 would slow down andstop. In order to prevent just this occurrence and at the same time tomaintain maximum sensitivity to slow down requirements, resistors 52 and52a which are equal in value to resistors 51 and 51a, diodes 54 and 54a,and failure sensitive switches 58 and 58a are included.

For example, failure sensitive switch 58 functions to short out thevoltage across Hall effect device 50 in the event that buffer amplifier46a output voltage is negative and exceeds 7 volts, the breakdownvoltage of Zener diode 62. Gain setting circuit 48 includes a firstcurrent path through resistor 51 to Hall effect device 50 with positiveoutput voltages from buffer amplifier 46 and a second current path ofequal impedance through resistor 52 and diode 54 on the appearance of anegative output voltage from buffer amplifier 46. Thus the samemagnitude output signal is applied to Hall effect device 50 with a 6volt negative output of buffer amplifier 46 as for a 12 volt positiveoutput. Thus the circuit has a conductance with respect to negativesignals of twice that for positive signals. Hence the amount ofdecelerating torque for a given negative signal will be twice theaccelerating torque for the same magnitude positive signal. In the eventthat because of circuit failure a negative voltage up to approximately 6volts should appear at the output of buffer amplifier 46 or 46a ofcontrol system 10 or 12, respectively, the resulting slow down would besensed by the speed transducer of the other control channel and a fullpositive 12 volt output of the buffer amplifier of that channel would beapplied through the appropriate gain setting circuit to the appropriateHall effect device to increase the forward current in one winding tooffset the reverse current in the other winding.

In the event that there is such a component failure that there appearsan undesired negative output of one of the buffer amplifiers in excessof 7 volts, the appropriate failure sensitive switch is activated byZener diode 62 and current limiting resistor 61 and shunts outthecurrent to the Hall effect device across which it is connected andthe other control channel senses the resulting slow down and increasescurrent to the winding to which that control channel is connected tomaintain speed. Thus, except for a small range of approximately -6 to 7volt outputs from a buffer amplifier, the system will correct for otherpossible abnormal voltages and maintain proper speed.

Similarly, if a component failed in such a manner as to cause either apartial or full acceleration torque to be applied by one of thechannels, the increase in speed would be sensed by the speed sensor inthe good channel. Since the feedback signal is now greater than thereference signal 40, the output of comparator 24 reverses its output andcauses negative current to flow in Hall effect device 50 and a negativeor decelerating compensating torque to be applied. Since the torque gainin the deceleration mode is twice that for acceleration and sincefriction aids in the deceleration mode the output of buffer amplifier 46never has to go greater than 6 volts negative to compensate for hardover failures in the positive direction. Hence hard over failures orfailures greater than 6 volts in the negative direction can be sensed byZener diode 62 and the effect of the failure eliminated by failure senseswitch 60.

From the foregoing it will be appreciated that this invention providesan improved system for the control of speeds of Hall effect deviceequipped motors enabling a degree of certainty and reliability of speedcontrol not previously available.

What is claimed is:

l. A motor control system for a Hall effect commutating motor havingfirst and second stationary windings, a permanent magnet rotor, andfirst and second Hall effect devices responsive to said rotor forcontrolling current fiow to said windings, comprising:

first and second speed signal generating means, each comprising meansresponsive to the rotation of said rotor for generating a signal of amagnitude proportional to the speed of said motor;

first and second comparator means, each comprising means responsive tothe output of a discrete said speed generating means for generating anoutput of a first polarity upon receiving a said signal of smaller valuethan a selected reference value and of an opposite, or second, polaritywhen said signal is of greater value than said selected reference value;

first and second failure sensitive coupling means,

each interconnecting the output of a discrete said comparator means to acontrol input of a discrete said Hall effect device and comprising meansresponsive to a preselected said value of said last named output forreducing its level to substantially zero;

first and second power supplies, said first power supply beingresponsive to a control output of said first Hall effect device forpowering said first winding and said second power supply beingresponsive to a control output-of said second Hall effect device forpowering said second winding;

whereby, said first speed signal generating means,

said first failure sensitive coupling means, said first Hall effectdevice and said first power supply comprise a first control channel, andsaid second speed signal generating means, said second comparator means,said second failure sensitive coupling means, said second Hall effectdevice and said second power supply comprise a second control channeland in the event of a failure of one of said control channels to providean output as indicated by a properly functioning said speed signalgenerating means, said other control channel will provide a correctiveoutput to correct the speed of said rotor.

2. A motor control system as set forth in Claim 1 wherein a said failuresensitive coupling means comprises:

a transistor switch connected across the control input of said Halleffect device; and

voltage sensitive means responsive to said preselected value of saidoutput from said comparator means for triggering on said transistorswitch and substantially shorting the control input to said Hall effectdevice.

3. A motor control system as set forth in Claim 2 further comprising again setting circuit interconnecting each said comparator means to eachsaid Hall effect device and comprising first and second impedance pathsconnected in parallel and wherein one of said impedance paths includes arectifier whereby current flow between a said comparator means and saidHall effect device in one direction is substantially greater thancurrent flow in the opposite direction.

4. A motor control system as set forth in Claim 3 wherein said impedancepaths have a conductance'to enable substantially identical current flowwith an applied one half voltage of a said second, or decrease speedpolarity compared with a first or increase speed" polarity signal fortriggering on" said transistor switch.

5. A motor control system as set forth in Claim 2 further comprising abuffer circuit interconnecting each of said comparators to a said gainsetting circuit, a said failure sensitive means and a said Hall effectdevice comprising an amplifier including a pair of complementarytransistors, the base input terminals of which are connected togetherand the emitter output terminals of which are connected together.

6. A motor control system as set forth in Claim 5 wherein:

each said speed signal generating means is an alternating currentgenerator; and

the system further comprises an input circuit interconnecting said speedsignal generating means of a said comparator means comprises anadjustable resistor, a rectifier, and a smoothing filter;

whereby a selectively attenuated direct current is provided as saidinput signal to a said comparator means.

1. A motor control system for a Hall effect commutating motor having first and second stationary windings, a permanent magnet rotor, and first and second Hall effect devices responsive to said rotor for controlling current flow to said windings, comprising: first and second speed signal generating means, each comprising means responsive to the rotation of said rotor for generating a signal of a magnitude proportional to the speed of said motor; first and second comparator means, each comprising means responsive to the output of a discrete said speed generating means for generating an output of a first polarity upon receiving a said signal of smaller value than a selected reference value and of an opposite, or second, polarity when said signal is of greater value than said selected reference value; first and second failure sensitive coupling means, each interconnecting the output of a discrete said comparator means to a control input of a discrete said Hall effect device and comprising means responsive to a preselected said value of said last named output for reducing its level to substantially zero; first and second power supplies, said first power supply beinG responsive to a control output of said first Hall effect device for powering said first winding and said second power supply being responsive to a control output of said second Hall effect device for powering said second winding; whereby, said first speed signal generating means, said first failure sensitive coupling means, said first Hall effect device and said first power supply comprise a first control channel, and said second speed signal generating means, said second comparator means, said second failure sensitive coupling means, said second Hall effect device and said second power supply comprise a second control channel and in the event of a failure of one of said control channels to provide an output as indicated by a properly functioning said speed signal generating means, said other control channel will provide a corrective output to correct the speed of said rotor.
 2. A motor control system as set forth in Claim 1 wherein a said failure sensitive coupling means comprises: a transistor switch connected across the control input of said Hall effect device; and voltage sensitive means responsive to said preselected value of said output from said comparator means for ''''triggering on'''' said transistor switch and substantially shorting the control input to said Hall effect device.
 3. A motor control system as set forth in Claim 2 further comprising a gain setting circuit interconnecting each said comparator means to each said Hall effect device and comprising first and second impedance paths connected in parallel and wherein one of said impedance paths includes a rectifier whereby current flow between a said comparator means and said Hall effect device in one direction is substantially greater than current flow in the opposite direction.
 4. A motor control system as set forth in Claim 3 wherein said impedance paths have a conductance to enable substantially identical current flow with an applied one half voltage of a said second, or ''''decrease speed'''' polarity compared with a first or ''''increase speed'''' polarity signal for ''''triggering on'''' said transistor switch.
 5. A motor control system as set forth in Claim 2 further comprising a buffer circuit interconnecting each of said comparators to a said gain setting circuit, a said failure sensitive means and a said Hall effect device comprising an amplifier including a pair of complementary transistors, the base input terminals of which are connected together and the emitter output terminals of which are connected together.
 6. A motor control system as set forth in Claim 5 wherein: each said speed signal generating means is an alternating current generator; and the system further comprises an input circuit interconnecting said speed signal generating means of a said comparator means comprises an adjustable resistor, a rectifier, and a smoothing filter; whereby a selectively attenuated direct current is provided as said input signal to a said comparator means. 